1. Field of the Invention
The present invention pertains to an apparatus and method for the maintenance of valve closure element/valve seat positioning within valves having trunnion mounted valve elements. More particularly, this invention relates to an apparatus and method to aid in the maintenance of disc/seat concentricity within a butterfly valve.
2. Description of the Prior Art
In valves in which the valve closure element is mounted by means of a trunnion or shaft within the valve body, misalignment of the valve closure element with the valve seat can result in accelerated seat wear, excessive leakage, and in aggravated situations, total valve failure. This is particularly true of butterfly valves, especially butterfly valves of double offset disc construction, with wetted shafts where line pressure on the lower enclosed end of the shaft can move the disc out of concentricity with the seat.
Disc/seat misalignment may also occur as a result of fitting an actuator to the valve. Since it is desirable that the shaft of the valve fit tightly in the actuator bore, it is necessary to exert some degree of force to push the actuator onto the valve shaft. If the disc is not restrained against downward movement as the actuator is fitted to the top of the valve, it will be displaced and moved out of concentricity with the seat.
Current solutions to the problem of preventing valve element/valve seat misalignment generally involve machining operations that bring the valve element, e.g. disc, and seat within close tolerances prior to assembly, or additional machining after assembly. One solution in the cse of butterfly valves has been to utilize spacers fitted between the disc hubs and the body bore. This solution demands the application of additional body machining to close tolerances or the use of sized spacers for individual valves. Either of these two methods is expensive. Furthermore, the end result is not particularly effective. Fitting spacers between the disc hubs and the body bore subjects the disc to jamming due to line media contaminants clogging, or corroding, the rubbing surface between the disc hubs and the spacers. This method also is very time consuming and does not lend itself to efficient manufacturing operations.
A second alternative solution utilizes a shaft incorporating an integral, nonadjustable end button. In this method, the valve is assembled and the disc centered prior to being pinned to the shaft. The valve then requires a further machining operation in order to fit the disc/shaft pins. This method involves handling and machining after the initial assembly which adds to costs and leads to inefficient manufacturing operations. Furthermore, this method exposes the assembled valve to undesirable metal cuttings from the machining operations.
If the same above method is used, but the disc and the shaft are already pre-drilled for pinning, the machining tolerances on the disc, the shaft, and the body must be held to extremely close limits.